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In the commercial swimming pool industry, the overwhelming choice for pool piping material selection is Polyvinyl Chloride (PVC). The use of such materials can provide security, as it is non-corrosive, and if installed properly, some would say this pipe material can provide an almost infinite life span. But, in comparison to other pipe material selections, such as cast-iron, ductile iron, steel and concrete, PVC has a much higher coefficient of thermal expansion. This coefficient considers the amount of expansion or contraction that will occur due to the temperature range your pipe will endure and the length of pipe you are calculating. Thus, the design and installation of PVC pipe must consider how to accommodate such changes in pipe length.
For pools located in areas with seasonal temperature changes that utilize long runs of straight pipe sections, consideration should be given to accommodating expansion and contraction of the pipe. The ASTM Standard 2774 Underground Installation of Thermoplastic Pressure Piping, contains specific information on the topic of expansion and contraction in pool piping. A science-based formula for determining the expected change in pipe length due to expansion and contraction is:

By knowing the amount of expansion or contraction that will occur in your piping system, you can adjust the design as needed. These accommodations can range from changes in vertical or horizontal direction of your piping system, to the use of mechanical expansion and contraction joints. Changes in pipe direction using expansion loops, offsets and bends are ways to accommodate the expected changes in pipe length within your system. Considering that pool piping systems often have changes in direction due to the inclusion of supply inlets, main drains and feature supplies, the design and installation of the underground pool piping system naturally accommodates expansion and contraction. However, there are times when pipe runs become quite lengthy without changes in direction, and considerations for the inclusion of an expansion loop or mechanical joint will be needed.

Mechanical expansion joints come in many different types. Their primary purpose is to provide a means of flexibility in the piping network for expansion and contraction. They often work by allowing the pipe to slide into or out of itself like a piston. Installation of mechanical expansion joints for underground piping is critical. If the mechanical expansion joints, along with the materials used for backfill around the joints, are not properly installed, the effectiveness can be compromised. For example, backfill materials can make their way into the mechanical joint and hamper pipe movement. If backfill materials are a concern, it is recommended to boot the joint for protection.

In most installations, the straight pipe runs are not excessive enough or the design of the piping network will already include many bends or turns in the pipe. However, for those occasions where environmental factors result in expansion and contraction of pool piping, or straight pipe runs 100 feet or more exist, the design and installation must have allowances for the changes in pipe length that will occur. Without these provisions, the underground piping will be susceptible to potential damage, which will result in leaks to your pool piping system.